NERVOUS CONDUCTION AND EXCITATION 147 



following immediately after a preceding one would be character- 

 ised by phenomena analogous to the refractory and supernormal 

 phases described above. Such has been shown to be the case 

 by Keith Lucas and Adrian. The method for measuring 

 the conductivity of a nerve after the passage of a previous 

 impulse was the same as that employed for demonstrating the 

 all-or-nothing law. The passage of a nervous impulse is 

 followed first by a refractory period in which the impulse is 

 more easily extinguished by narcosis, and then by a supernormal 

 phase in which it is able to traverse a longer distance through 

 a region of decrement than in the resting state. These facts 

 are of great interest in connexion with the phenomena of 

 summation in the central nervous system and in peripheral 

 tissues. 



Summation in Peripheral Tissues.— So far we have dealt 

 exclusively with excitation and conduction in nerve-trunks. 

 The phenomena of propagation of nervous impulses in reflex 

 arcs show two outstanding peculiarities differing — one in kind, 

 the other only in degree — from the phenomena of nervous 

 transmission hitherto described. When a synapse intervenes 

 in the path of the nervous impulse, conduction can take place 

 only in one direction — from the afferent to the efferent side. 

 Furthermore, a single stimulus of any strength whatever usually 

 proves inadequate to induce a response, although sufficient 

 to set up a disturbance (measured by the electrical variation) 

 in the afferent nerve. Thus Sherrington, to whose labours 

 we owe so much of our knowledge of reflex mechanisms, 

 records cases in which the scratch reflex (alternate responses 

 of the flexors and extensors of the hind-limb on stimulating 

 an area of the back in the spinal dog) did not appear till the 

 fortieth shock had been administered. The significance of 

 this phenomenon receives some light from a consideration of 

 the phenomena of summation in peripheral tissues. We have 

 already considered a condition in the sciatic gastrocnemius 

 preparation of the frog in which the minimal intensity for a 

 second stimulus is lower than for the initial one. This depends 

 upon the mechanism of excitation at the seat of application 

 of the stimulus : a probable explanation is that summation 



